1. Field of the Invention
The present invention relates generally to a common channel communication apparatus and method for a Code Division Multiple Access (CDMA) communication system, and in particular, to an channel assignment apparatus and method for common packet channel in a wideband CDMA (W-CDMA or WCDMA) mobile communication system.
2. Description of the Related Art
Wideband CDMA communication systems, such as the UMTS (Universal Mobile Telecommunications System) W-CDMA (Wideband Code Division Multiple Access) communication system, which is a planned future mobile communication system, use a random access channel (RACH) and a common packet channel (CPCH) for an uplink (or reverse) common channel.
FIG. 1 is a diagram for explaining how to transmit and receive a message over the RACH, which is one of the uplink common channels, in a W-CDMA communication system.
In FIG. 1, reference numeral 151 indicates a signal transmission procedure of an uplink channel, for which the RACH may be used. The RACH is one of the common channels, over which a user equipment (UE, or a mobile station) transmits a signal to a UMTS terrestrial radio access network (UTRAN, or a base station). Further, reference numeral 111 indicates a signal transmission procedure of a downlink (or forward) channel, for which an access preamble-acquisition indication channel (AICH) may be used. The AICH is a channel over which the UTRAN responds to a preamble upon receipt of the preamble signal transmitted over the RACH. The preamble transmitted over the RACH is an access preamble (AP), which is created by selecting one of the signatures for the RACH.
The RACH consists of a preamble part and a message part. To transmit a message over the RACH, the UE selects an access service class (ASC) according to the type of transmission data, selects a RACH sub-channel group, which is defined in the ASC, and transmits the AP to the UTRAN at the selected RACH sub-channel. Thereafter, the AP signal is acquired by the UTRAN. The UTRAN response to the AP signal over the AICH. If the UE receives an acknowledgment AICH signal from the UTRAN, the UE transmits the RACH message part signal to the UTRAN.
Referring to FIG. 1, the UE transmits an AP of specific length using a signature, represented by 162, and then awaits a response from the UTRAN for a predetermined time τP-P. If there is no response from the UTRAN for the predetermined time τP-P, the UE increases transmission power by a specific level as represented by 164 and retransmits the AP at the increased transmission power. Upon detecting the AP transmitted over the RACH, the UTRAN transmits a signature of the detected AP after a predetermined time τP-AP—A1, represented by 122, over the AICH for the downlink. After transmitting the AP, the UE examines the AICH in order to detect the signature used for the AP. If the signature used for the AP transmitted over the RACH is detected, the UE judges that the UTRAN has detected the AP, and transmits a RACH message and control part after a predetermined time τAP—AI-MSC, represented by 170, over the RACH.
Otherwise, upon failure to receive the AICH signal transmitted from the UTRAN within a set time (τp-p) after transmission of the AP 162, or upon failure to detect the transmitted signature from the received AICH, the UE judges that the UTRAN has failed to detect the AP, and retransmits the AP after a lapse of a preset time (τp-p). At this point, the AP is retransmitted at a transmission power increased by ΔP (dB), represented by 164, as compared with the transmission power at which the AP was previously transmitted. For the retransmitted AP, a signature can be used which is randomly selected from the signatures defined in the ASC selected by the UE. If the AICH signal using the signature transmitted by the UE itself is not received from the UTRAN after transmission of the AP, the UE changes, after a lapse of a set time (τP-P), the transmission power and signature of the AP and repeatedly performs the above operation. When the AICH signal is received and if the signature transmitted by the UE itself is received, the UE spreads, after a lapse of a preset time (τP-AP—A1), the RACH message 170 with a scrambling code for the signature, and transmits the spread RACH message using a predetermined channelization code at transmission power which will be determined with consideration of the AP transmission power.
As described above, it is possible for the UTRAN to efficiently detect the AP and to readily set the initial power of an uplink common channel message by using the AP. However, the uplink common channels, such as the RACH, are not power controlled channels. Accordingly it is very difficult for the uplink common channel to transmit a packet data because the packet data has a long transmission time or requires a high data rate. In the long transmission time or the high data rate transmission, the power control is essential to transmit data without an error. In addition, since the UTRAN allocates the RACH through one AP_AICH (Access Preamble Acqusition Indicator Channel), the same channel may be allocated to several UEs that have transmitted the AP using the same signature. In this case, the data transmitted by the different UEs collide with one another, so that the UTRAN cannot receive the data.
To solve this problem, a method for suppressing a collision between the UEs while power controlling the uplink common channel has been proposed for the W-CDMA system. This method is called the CPCH (Common Packet Channel). The CPCH enables power control of the uplink common channel, and shows a high reliability as compared with the RACH in allocating the channel to different UEs. Further, the CPCH is a common channel over which the UE can transmit high rate data for a predetermined time (from several tens to several hundreds of ms). One purpose of using the CPCH is to enable the UE to quickly transmit an uplink transmission message, which is smaller in size than a specific value, to the UTRAN without using a dedicated channel.
That is, in order to establish a dedicated channel, many related control messages are exchanged between the UE and the UTRAN, and a long message transmission/reception time is required. Therefore, when the dedicated channel is allocated to transmit data of a small size (e.g., data of a comparatively small size of several tens to several hundreds of ms), exchanging the many control messages during channel assignment becomes a needlessly large amount of overhead. Thus, it is more effective to use the CPCH, when transmitting data of a small size.
However, since several UEs transmit preambles using several signatures in order to acquire the right of using the CPCH from the UTRAN, there may occur a collision between the UEs. To avoid this phenomenon, a method is needed for allocating the right of using the CPCH to the UEs.
The W-CDMA communication system uses a downlink scrambling code to distinguish the UTRANs, and uses an uplink scrambling code to distinguish the UEs. Further, the channels transmitted from the UTRAN are distinguished using an orthogonal variable spreading factor (OVSF) code, and the channels used in the UE are also distinguished using the OVSF code.
Therefore, the information required by the UE to use the CPCH, includes a scrambling code used for a message part of the uplink (or reverse) CPCH channel, an OVSF code used for the message part (UL_DPCCH) of the uplink CPCH, an OVSF code used for a data part (UL_DPDCH) of the uplink CPCH, a maximum data rate of the uplink CPCH, and a channelization code for a downlink (or forward) dedicated channel (DL_DPCCH) used for power control of the CPCH. The above information is typically required when establishing a dedicated channel between the UTRAN and the UE. Further, the above information is transmitted to the UE through transmission (overhead) of signals before establishment of the dedicated channel. However, since the CPCH is a common channel rather than a dedicated channel, the above information can be conventionally represented by a combination of the signatures used in the AP and the CPCH sub-channels, which is similar to the ASC used in the RACH, in order to allocate the information to the UE.
FIG. 2 shows a conventional signal transmission procedure of the downlink and uplink common channels. In FIG. 2, in addition to the method used for the RACH for transmitting the AP, a collision detection preamble (CDP) is used to prevent a collision between CPCH signals from the different UEs.
In FIG. 2, reference numeral 211 indicates an operating procedure of an uplink channel performed when the UE operates to be allocated the CPCH, and reference numeral 201 indicates an operating procedure of the UTRAN to allocate the CPCH to the UE. In FIG. 2, the UE transmits the AP 213. A signature constituting the AP 213 can be selected from a signature group used in the RACH or the same signature for the RACH. If the signature for CPCH is identical to the signature for RACH., the signature for CPCH can be distinguished from the signature for RACH by using different scrambling codes. The signature constituting the AP is selected by the UE based on the followed-stated information, and such a method is different from the method where the RACH randomly selects the signature. That is, onto each signature are mapped an OVSF code to be used for the UL_DPCCH, an OVSF code to be used for the UL_DPDCH, a UL_Scrambling code to be used for CPCH, an OVSF code for DL_DPCCH, the maximum frame number which indicates the length of data, and a data rate which indicates the data transmission speed. Therefore, selecting one signature is equivalent to selecting six kinds of the information mapped to the corresponding signature. In addition, the UE examines a status of the CPCH channel by using CSICH (CPCH Status Indicator Channel) before transmitting AP. The CSICH using an ending part of the AP13 AICH is transmitted by the UTRAN. The UE transmits the AP after selecting the signatures used for the CPCHS that are available at present. The AP 213 is transmitted to the UTRAN at initial transmission power set by the UE. In FIG. 2, if there is no response from the UTRAN within a time 212, the UE retransmits the AP 215 at a greater transmission power than that of the first AP transmission. Before the CPCH channel acquisition process, the retransmission number of the AP and the waiting time 212 are set, and the UE stops the CPCH channel acquisition process when the retransmission number exceeds a set value.
Upon receipt of the AP 215, the UTRAN compares the received AP with the APs received from other UEs. Upon selecting the AP 215, the UTRAN transmits AP_AICH 203 as ACK after a lapse of a time 202. There are several criteria based on which the UTRAN compares the received APs to select the AP. 215. For example, the criteria may correspond to a case where the CPCH, for which the UE has requested the UTRAN through the AP, is available, or a case where the receiving power of the AP satisfies the minimum receiving power requested by the UTRAN. The AP_AICH 203 includes a value of the signature that constitutes the AP 215, are received and selected by the UTRAN.
If the signature transmitted by the UE itself is included in the AP_AICH 203 after transmitting the AP 215, the UE transmits a collision detection preamble CD_P 217 after a lapse of a time 214. A reason for transmitting the CD_P 217 is to prevent a collision between the UEs. In other words, many UEs belonging to the UTRAN may request the right of using the same CPCH by simultaneously transmitting the same AP to the UTRAN, and as a result, the UEs receiving the same AP_AICH may try to use the same CPCH, thereby causing a collision. To prevent such a collision, the UE transmits the CDP and the UTRAN selects one UE among the UEs which had transmitted the same AP and have transmitted the different CD_P.
The detailed operation of the UE and UTRAN is as follows. To prevent a collision, each of the UEs which have simultaneously transmitted the same AP, selects the signature to be used for the CD_P and transmits the CD_P. Upon receipt of the CD_Ps, the UTRAN can select one of the received CDPs and respond to the selected CDP. For example, a criterion for selecting the CD_P can be a receiving power level of the CD_P received from the UTRAN. For the signature constituting the CD_P 217, one of the signatures for the AP can be used, and it can be selected in the same manner as in the RACH. That is, it is possible to select randomly one of the signatures used for the CD_P and transmit the selected signature. In addition, for the CD_P, only one signature can be set and used. When there is only one signature used for the CD_P, the UE transmits the CD_P at a specific time point during a certain time period. This method can distinguish the UEs which use one signature for CD_P but use a different transmission point.
Upon receipt of the CD_P 217, the UTRAN compares the received CD_P with the CD_Ps received from other UEs to select a UE which can use the CPCH. Upon selecting the CD_P 217, the UTRAN transmits a collision detection indicator channel (CD_ICH) 205 to the UE after a lapse of a time 206. CD_ICH has the same structure and function as AP_AICH in the RACH transmission. But CD_ICH transmits only one ACK. Upon receipt of the CD_ICH 205 transmitted from the UTRAN, the UEs check whether a value of the signature used for the CD_P transmitted by themselves (i.e., CD_ACK) is included in the CD_ICH 205, and the UE, for which the signature used for the CD_P is included in the CD_ICH 205, transmits a power control preamble (PC_P) 219 after a lapse of a time 216. The PC_P 219 uses an uplink scrambling code determined while the UE determines a signature to be used for the AP, and the same channelization code (OVSF) as a control part (UL_DPCCH) 221 during transmission of the CPCH. The PC_P 219 is comprised of pilot bits, power control command bits, and feedback information bits. The PC_P has a length of 0 or 8 slots. The slot is a basic transmission unit used when the UMTS system transmits a physical channel, and has a length of 2560 chips when the UMTS system uses a chip rate of 3.84 Mcps (chips per second). When the length of the PC_P 219 is 0 slots, the present radio environment between the UTRAN and the UE is good, so that there is no need to control transmission power of CPCH message part and the CPCH message part can be transmitted at the transmission power determined by the UE in consideration of the transmission power of CD_P. When the length of the PC_P 219 is 8 slots, it is necessary to control transmission power of the CPCH message part.
The AP 215 and the CD_P 217 may use the scrambling codes which have the same initial value but have different start points. For example, the AP can use oth to 4095th scrambling codes of length 4096, and the CD_P can use 4096th to 81915th scrambling codes of length 4096. The AP and CD_P can use the same part of the scrambling code having the same initial value, and such a method is available when the W-CDMA system separates the signatures used for the uplink common channel. into the signatures for the RACH and the signatures for the CPCH. For the scrambling code used for the PC 219, are used the 0th to 21429th values of the scrambling code having the same initial value as the scrambling code used for AP 215 and CD_P 217. Alternatively, for the scrambling code for the PC_P 219, a different scrambling code can also be used which is mapped one-to-one with the scrambling code used for AP 215 and CD_P 217.
Reference numerals 207 and 209 denote a pilot field and a power control command field, respectively, of a dedicated physical control channel (DL_DPCCH) which is a part of downlink dedicated physical channels (DL_DPCHs). The DL_DPCCH can use a primary downlink scrambling code for distinguishing the UTRANs and can also use a secondary scrambling code for expanding the capacity of the UTRAN. The channelization code OVSF to be used for the DL_DPCCH is a channelization code which is determined when the UE selects the signature for the AP. The DL_DPCCH is used when the UTRAN performs power control on the PC_P or CPCH message transmitted by the UE. The UTRAN measures receiving power of a pilot field of the PC_P 219 upon receipt of the PC_P, and controls transmission power of the uplink transmission channel transmitted by the UE, using the power control command 209. The UE measures power of a DL_DPCCH signal received from the UTRAN to apply a power control command to the power control field of the PC_P 219, and transmits the PC_P to the UTRAN to control transmission power of a downlink channel incoming from the UTRAN.
Reference numerals 221 and 223 denote a control part UL_DPCCH and a data part UL_PDCH of the CPCH message, respectively. For a scrambling code for spreading the CPCH message of FIG. 2, a scrambling code is used which is identical to the scrambling code used for the PC_P 219. For the used scrambling code, are used 0th to 38399th scrambling codes of length 38400 in a unit of 10 ms. The scrambling code used for the message of FIG. 2 can be either equal to the scrambling code used for the AP 215 and the CD_P 217, or a different scrambling code which is mapped one-to-one. The channelization code OVSF used for the data part 223 of the CPCH message is determined according to a method previously appointed between the UTRAN and the UE. That is, since the signature to be used for the AP and the OVSF code to be used for the UL_DPDCH are mapped, the OVSF code to be used for the UL_DPDCH is determined by determining the AP signature to be used. For the channelization code used by the control part (UL_DPCCH) 221, a channelization code is used which is identical to the OVSF code used by the PC_P. When the OVSF code to be used for the UL_DPDCH is determined, the channelization code used by the control part (UL_DPCCH) 221 is determined according to an OVSF code tree structure.
Referring to FIG. 2, the prior art enables power control of the channels in order to increase an efficiency of the CPCH and decreases the chance of a collision between uplink signals from the different UEs, by using the CD_P and the CDICH. In the prior art, the UE selects all the information for using the CPCH and transmits the selected information to the UTRAN. This selecting method can be performed by combining a signature of the AP transmitted from the UE, a signature of the CD_P and the CPCH sub-channel. In the prior art, the UE requests an assignment of the certain CPCH channel by analyzing CSICH which transmits the present status of CPCH in the UTRAN and the information is predetermined by the UE in consideration of the data transmitted over CPCH. That is, the assignment of CPCH depends on the UE and not on the UTRAN. Accordingly, even though the UTRAN has the CPCHs which have the same characteristic required by the UE, if the UE requires a specific CPCH the UTRAN cannot assign a CPCH to the UE. So this will cause a limitation in assignment of the CPCH channel and a delay in acquiring the CPCH.
The limitations in assignment of the CPCH channel are as follows. There exist several available CPCHs in the UTRAN. If the UEs in the UTRAN require the same CPCH, the same AP will be selected. Although the same AP_AICH is received and the CD_P is transmitted again, the UEs which transmitted the non-selected CD_P should start the process for allocating the CPCH from the beginning. In addition, although the CD_P selecting process is performed, many UEs still receive the same CD_ICH and are increasing a probability that a collision will occur during uplink transmission of the CPCH. Moreover, even if the CSICH is checked and the UE requests the assignment of the CPCH in consideration of the current CPCH status transmitted over the CSICH, all the UEs in the UTRAN which desire to use the CPCH receive the same CSICH. Therefore, even though an available channel is required out of the CPCHs, there is a case where several UEs simultaneously requests a channel assignment of a specific CPCH. In this case, the UTRAN cannot but allocate the CPCH requested by the several UEs to only one UE, even though there are other CPCHs which can be allocated. This is due to the channel assignment determined by the UE.
With regard to a delay in acquiring the channel, when the case occurs which has been described with reference to the limitations in assignment of the CPCH channel, the UE should repeatedly perform the CPCH assignment request to allocate the desired CPCH channel. When there is used a method for transmitting the CDP at a given time for a predetermined time using only one signature for the CD_P introduced to reduce the complexity of the system, it is not possible to process the CD_ICH of other UEs while transmitting and processing the CD_ICH of one UE.
In addition, in the prior art, one uplink scrambling code is used in association with one signature used for the AP. Thus, whenever the CPCH used in the UTRAN increases in number, the uplink scrambling code also increases in number, causing a waste of the resources.